The present invention relates to a useful pass band filter, which comprises a plurality of superconductor planar resonators and is suitable for use in high frequency communication and navigation systems.
Pass band filters are used in high frequency systems on the receiver side, e.g. as pre-selection filters and in the form of a filter bank for frequency channeling (input multiplexer). On the transmission side they form, e.g., elements of output multiplexers whose purpose is to provide the most loss-free transmission of amplified signals of the different frequency channels as possible to a common antenna.
These pass band filters are chiefly made from individual resonators that are coupled with each other coupled to and suitable feed lines. The function of the resonators in a pass band device is to provide an input of electromagnetic energy which is as loss-free as possible. The dissipation losses unavoidably connected with energy supply in resonators can be described quantitatively by the so-called non-load Q-factor. The non-load Q-factor, Q.sub.O, of a resonator gives the ratio of the product of the average field energy W and the circuit resonance frequency .omega..sub.O to the dissipated power
Pdiss: EQU Q.sub.O =.omega..sub.O W/P.sub.diss.
Dissipation losses degrade the frequency transmission path through the pass band filter in comparison to an ideal loss-less pass-band filter so that the attenuation in the pass band range is increased and the pass band sides are "worn away". The smaller the relative bandwidth of the filter and the steeper its pass band sides, the greater this degrading effect of the dissipation losses is. Resonators of comparatively higher Q-factor, typically Q.sub.O &gt;10000, are required for filters with comparatively high specifications for pass band side steepness and relative bandwidth. The smaller the geometric dimensions of the individual resonators, the smaller the desired Q-factor, considering filters of different structural forms made from standard conductors, e.g. filters made from hollow resonators, from coupled coaxial resonators or from coupled planar microstrip resonators. Thus filters having high specifications must be made from relatively large hollow resonators.
Resonators can be made which have Q-factors up to about 200,000 and operate at operating temperatures of about 60 to 80 K by using cooled planar resonator structures with conductor strips made from high temperature superconductors on crystalline substrate materials and with substantially smaller geometric dimensions than conventional hollow resonators with Q-factors of about 20,000.
The use of planar resonators made from high temperature superconductors in filters for "high" operating powers is however limited by physical principles so that the superconductor properties of the currently known materials are degraded when the magnetic field strength of the high frequency field exceeds a value of about 50 A/cm at the surface of the superconductor film. This effect has proven to be especially disadvantageous in planar conducting structures, since a local field increase of about a factor of 10 occurs for current lines extending with their edges parallel because of magnetic field penetration at the edges of the conductors. The value of the maximum high frequency magnetic field strength is proportional to the square root of the field energy supplied to the resonator, which depends on the proportionality factor of the resonator shape and the oscillator type. Furthermore the field energy supplied per resonator is proportional to the throughput power of the filter and the characteristic value of the relative bandwidth.
The filter properties are already degraded because of the above-described effects using superconductor planar resonators with edge-parallel current flow lines in filters with a relative bandwidth on the order of about 0.3 to 2%, when the operating power exceeds a value of about 0.2 to 2 W.
One solution to the problem of the lower energy capacity of planar resonators made from a high temperature superconductor is disclosed in the invention described in German Patent Application DE 44 36 295 A1. The solution suggested there provides for use of a circular disk- or ring resonator, which is exited in a TM010-oscillator mode. Since no edge-parallel current flow lines occur, about a factor of 100 higher electromagnetic field energy can be fed to this resonator in comparison to a resonator of the same volume but with edge-parallel currents. For a filter with a bandwidth of about 0.3 to 2% a power compatibility of at least 20 to 200 W is achieved with this type of resonator.
Resonators comprising a crystalline substrate with a thin superconductor film grown on both sides are described in German Patent Application DE 44 36 295 A1. On one side designated the "front side" the superconductor layer is structured so that only one circular conductor surface or only one concentric ring-shaped conductor surface remains. On the other side, designated as the "rear side", the conducting layer extends up to the substrate edge. However circular or ring-shaped openings in the conducting layer for coupling purposes are provided on this rear side according to DE 44 36 295 A1. In this patent application a pass band filter is made from resonators arranged partially over each other and partially beside each other.
There are additional requirements for this type of pass band filter described in DE 44 36 295 A1 when it is used in output multiplexers (e.g. for communication satellites) which require solution of additional technical problems beyond those solved by the invention described in DE 44 36 295 A1. These technical problems and their solution are described here. The present invention at least partially results from the solution of these technical problems and is clearly distinguished in an outstanding manner from the state of the art.
In a case in which the pass band filters are used in an input or output multiplexer, different throughput frequencies are assigned to the individual filters that together determine the operating frequency range of the multiplexer. The typical relative bandwidth of an individual filter amounts to about 1%, while the entire operating frequency range has a typical width of about 20%. This means that substantially no degradation of the blocking properties in the entire frequency range above a pass band with a width of about 20% may occur for a filter with a bandwidth at the lower end of the operating frequency range. This frequency range is designated as the "operating blocking range" in the following discussion. In an analogous manner a pass band with a width of about 20% must be free of interference of the blocking properties for a filter with the pass band at the upper end of the operating frequency range.
All resonators have additional undesired oscillator modes ("interfering modes") at other frequencies in addition to the desired oscillator modes. The edge-current-free TM010-oscillator mode desired for operation of the filter does not represent a fundamental oscillator mode and thus there are both undesired oscillator modes with resonance frequencies above the resonance frequency of the TM010-mode and also undesired oscillator modes with resonance frequencies below the resonance frequency of the TM010-mode. The adjacent oscillator mode in the frequency range with a lower resonance frequency is the TM210-oscillator mode and the adjacent oscillator mode of higher frequency is the TM310-oscillator mode. The spacing of the resonance frequencies of these oscillator modes depends on several geometric parameters. The block properties of a filter are degraded when the resonance frequency of an undesired oscillator mode falls in the operating blocking range. No solution for these problems is described in German Patent Application DE 44 36 295 A1.
The required resonance frequencies and coupling factors between the individual resonators are derived from the specifications for the pass band filter to be constructed. In filter design these set values or desired values are converted into geometric structural parameters ("design values"). The filter resulting from this filter design process has however properties deviating from the desired frequency behavior because of approximations in the theoretical modeling and because of manufacturing tolerances and material variations. In filters with comparatively small bandwidth the filter must contain tuning elements which allow a subsequent fine correction ("trimming") of the filter parameters.
It is advantageous when the resonance frequencies of the individual resonators can be trimmed separately from each other, i.e. individually, and also when the coupling factors between the resonators are changeable by mechanical means. In the structure of the pass band filter proposed in German Patent Application DE 44 36 295 A1 all the front side of the planar resonators arranged over each other are oriented on the same side and the front side of a resonator under another resonator is coupled with the other resonator by coupling holes or coupling rings in the rear side of the other resonator. Dielectric tuning screws or other dielectric inserts are placed in the space between the two resonators so that a displacement of this dielectric insert causes an equalization of the resonance frequency of the resonator and the coupling factor.
In DE 44 36 295 A1 the coupling between the gates and resonators and the coupling of resonators beside each other is accomplished by the standard structures known in the art. This type of coupling can lead to a degradation of the Q-factor by dissipation losses in the coupling elements.